Momma Robotnik PERIOD. She constantly abuses her son (Robotnik!) and is always escaping from a mental institution.
A mental institution known as "Mobius Home for Really Bizarre Mothers" no less.
In episode 38, we get this wonderful gem: (Robotnik has just completed his own robot wife) When Robotnik revealed her to Scratch and Grounder, Grounder commented that "She's got really big..." Scratch interrupts him saying "...hair!" Getting Crap Past the Radar much?
The "Zoobotnik" episode takes the cake for the best (and cruelest) prank Sonic has ever played on Robotnik. He disguises himself as a justice of the peace, marries Robotnik to Katella, and invites Mama Robotnik to the wedding. Hilarity Ensues.
At the beginning of "Black-Bot the Pirate", after Scratch accidentally uses Robotnik's time machine to go back to the first Thanksgiving, he comes back after apparently ending up in their turkey and complains "Stupid pilgrims! It's bad enough they tried to eat me for dinner, then they had the nerve to say I tasted lousy!".
Pretty much the entirety of "Musta Been a Beautiful Baby."
The entirety of the breakfast scene from the beginning of "Robotnikland".
Scratch: WE ARE GOING TO DIE! WE ARE GOING TO DIE! *crash* Scratch: ...WE ARE DEAD!
This exchange a mite earlier in the same episode:
Scratch: You will stop in two seconds flat! Hypnotized Road Hog: I will stop you in two seconds with a flat! He proceeds to give Scratch's vehicle a flat tire, thereby stopping him in two seconds with a flat.
From the opening of "Birth of a Salesman":
Robotnik: Nincombots! Metal morons! Idiots! Robotic rejects! Seditious slabs of unsalubrious circuitry! You...you...you nincombots! Scratch: Excuse me, Your Viciousness, you already said that. Robotnik: Then allow me to repeat myself yet again! YOU! ARE! NINCOMBOTS!
In the German dub, Scratch Screams Like a Little Girl. In the first episode, when Grounder drills him in the tail, Scratch lets out a ridiculously high-pitched shriek of pain.
Also in the same episode, when Sonic tricks Scratch and Grounder into being smashed by posing as a post man with fan letters (he actually used it twice in the episode), a mushed Grounder responds with this.
Robotnik's interview in "Lifestyles of the Sick and Twisted".
And then there's the ever-classic "SnooPING AS usual, I see!"
In "Over The Hill Hero", after Sonic uses Captain Rescue's rescue rocket to go up into space so the force field he's trapped in can release him (thanks to air pressure which Sonic and Captain Rescue pointed out themselves), Sonic ends up hanging around in space.
During the episode, "Boogey-Mania", we get this exchange;
Robotnik: Fools! Idiots!
Scratch and Grounder: You called?
The part in "Momma Robotnik's Birthday" where Sonic tricks Scratch and Grounder into thinking they're on a game show and as Robotnik is about to tell them on the phone that the game show host is actually Sonic, Sonic slices the phone cord with a knife, which somehow cuts the line on both ends much to Robotnik's bewilderment.